MINOT, N.D. (AP) — The military believes an engine that dropped from a B-52 bomber during a training mission this week in rural North Dakota suffered a “catastrophic failure,” disintegrated and fell into a river, Air Force Secretary Deborah Lee James said.

An in-flight emergency was declared Wednesday and the crew landed the plane safely, according to Minot Air Force Base. No injuries were reported on the ground or among the plane’s five crew members. No weapons were on board, according to the military.

A B-52 has eight engines, according to Boeing, the plane’s manufacturer.

The Air Force said the engine fell in a remote area about 25 miles northeast of the base that’s near Minot, a city about 110 miles north of Bismarck.

“They do believe at this point that it was really a catastrophic failure of that one engine,” James told the Minot Daily News. “Another way of putting that is it disintegrated and then fell out of the aircraft upon disintegration.

“They think it’s at the bottom of a river, so recovery of that is something we would like to do because that helps in understanding what happened,” she said.

Dom Christianson, who lives in Upham, told KXMC-TV that he saw several military helicopters flying over the J. Clark Salyer National Wildlife Refuge northeast of Minot. The Souris River runs through the refuge.

“Average day until about noon, when we had helicopters flying over town and flying really low out across the refuge out here,” he said. “It went on for about four hours.”

Former National Transportation Safety Board chairman and retired Air Force Maj. Gen. Mark Rosenker told KFGO-AM radio that the incident is “absolutely extraordinary.”

James said there is “zero indication” the engine incident is a widespread problem within the B-52 fleet.

“There’s no evidence of that. It appears this was a one-off situation, but again I want to emphasize it just happened and it will be fully investigated,” James said.

The investigation is expected to take months, Air Force officials said.